Delta’s Carbon Council is also working across Delta’s ACS, Flight Ops and Delta Connection teams to help reduce APU run-time between flights. APU refers to the auxiliary power unit, or the small engine at the back of the plane that helps provide power and temperature control while the plane is on the ground. Each minute of system-wide APU operation accounts for 1.2 million gallons of fuel consumption per year, so reducing usage can lead to huge fuel savings for Delta.